(1) Field of the Invention
The present invention relates to imaging processing performed by an ultrasound diagnostic device, and in particular to hardness measurement using a shear wave.
(2) Description of the Related Art
Recent years have seen the widespread use of ultrasound diagnostic devices that have the function of evaluating the hardness of a tissue inside the subject. There are roughly two methods for evaluating the hardness using an ultrasound diagnostic device. One method is to apply pressure to a tissue inside the subject from the body surface by using an ultrasound probe, release the pressure, and then evaluate the relative hardness of the tissue inside the subject based on the magnitude of distortion of the tissue inside the subject relative to the pressure. With this method, it is possible to evaluate a relative hardness inside the subject, which indicates whether the tissue is hard or soft compared to surrounding tissues.
The other method is to cause a region of interest (ROI) inside the subject to generate a shear wave, and measure the propagation speed of the shear wave by obtaining the displacement of the tissue within the region of interest in a time series. Since the propagation speed of the shear wave changes depending on the elastic modulus of the tissue, this method makes it possible to evaluate the absolute hardness of the tissue. As a technique to generate the shear wave, a technique called acoustic radiation force impulse (ARFI) can be used for example, by which the tissue inside the subject is displaced by the sound pressure of ultrasound.
With this method, however, there are cases where the speed of the shear wave cannot be properly calculated. One example is the case where the displacement of the tissue within the region of interest is irregular. In such a case, a displacement due to a movement of the entire tissue cannot be accurately extracted, and accordingly, the displacement due to the movement of the entire tissue is mixed in with a displacement due to the shear wave, and the accuracy in calculating the speed of the shear wave decreases. Another example is the case where the shear wave undergoes deflection or reflection at the boundary of the tissue. Conventionally, the speed of a shear wave is calculated on the assumption that, for each of several points within a tissue, the time at which the displacement reaches its maximum for the first time is the arrival time of the first wavefront of the shear wave arriving at the corresponding point. That is, the method is predicated on the case where a shear wave travels away from its source, and therefore, in other cases, the speed of the shear wave cannot be calculated, or the calculation results are inaccurate.
In order to address such a problem, when the arrival time of a shear wave at a given point within the tissue is significantly different from the arrival times at its surrounding points, or when the speed of the shear wave cannot be calculated, it is conventionally proposed to refrain from calculating the speed of the shear wave at the given point or inform the examiner that the speed of the shear wave at the given point has low reliability. Also, according to the technique disclosed in Japanese Patent Application Publication No. 2014-260, the arrival time distribution of the shear wave is shown on a shear wave arrival image, in which hues are allocated according to the arrival time of the shear wave, and thus the speed of the shear wave is visually shown, and also regions where the reliability of the speed of the shear wave is low are explicitly shown.